In a digital oscilloscope, or in a similar electronic instrument that digitizes and displays electronic waveforms, analog signals are sampled, the analog samples are then digitized by an analog-to-digital (A/D) converter, and the digital samples are stored in an acquisition memory (also called a capture memory). The digital samples are then processed (for example, perhaps filtered or interpolated) and the processed samples are written to a display memory or displayed directly. As will be discussed in more detail below, various trade-offs may be required among acquisition time window, display sample rate, display bandwidth and display update rate.
For the horizontal axis of the display, the operator of an oscilloscope (analog or digital) typically selects time-per-division (for example, 1.0 microsecond per horizontal centimeter on the face of the display). The input signal is then sampled for a total time (the acquisition time window) equal to the total time represented by the display. The number of acquisition samples (the acquisition record) is determined by the acquisition sample rate (samples per second) multiplied by the acquisition time window (seconds). Acquisition memory may be limited so that for some very long acquisition time windows a fraction (for example, every Nth sample) of the acquisition samples is stored. The maximum possible rate at which the display can be updated (display update rate) is the inverse of the operator selected acquisition time window. For example, if the operator selected acquisition time window is 0.01 seconds, the maximum display update rate is 100 updates per second. As discussed in more detail below, the actual display update rate may be less than the maximum.
Typically, digital oscilloscopes and similar waveform acquisition instruments can acquire and store data records much faster than they can process and display the records. For example, currently available analog-to-digital (A/D) converters can sample and convert a signal at the rate of several Gigasamples/second whereas currently available video processors can process these samples at a rate of several Megasamples per second. Assume that for a particular operator selected acquisition time window there are M acquired samples (acquisition record length) and N display samples (display record length) with M&gt;N. One traditional solution is to display every (M/N)th acquired sample, selecting acquisition samples or interpolated acquisition samples by a process called decimation. Note, in this patent document, decimation is used generally to designate a system or filter with an input sample rate greater than or equal to an output sample rate. For example, if the instrument acquires 10,000 samples and displays 2,000 samples, every 5th acquisition sample may be displayed. In general, decimation may include digital filtering so that, for example, if the instrument acquires 10,000 samples and displays 3,000 samples, the displayed samples may be obtained by filtering/interpolation. Typically, for digital oscilloscopes, the display record length is fixed.
Ideally, the display sample rate is sufficiently high to approach continuous waveform display as in an analog oscilloscope. The term "bandwidth" usually refers to the highest frequency that can be reproduced in sampled form, which according to basic sampling theory is one half the sample rate (also known as the Nyquist rate). If the acquisition samples are decimated for display, then there is an acquisition bandwidth and a separate display bandwidth. The display record length is the display sample rate multiplied by the operator selected acquisition time window. Typically, the display record length is fixed and the display bandwidth is variable as determined by the operator selected acquisition time window. Therefore, in general, there is a need for very long display record lengths to permit high display bandwidths for all operator selected acquisition time windows.
Ideally, the display update rate is high to maximize the amount of information presented to the operator. A high display update rate is particularly important for capturing intermittent events and amplitude varying signals. As discussed above, the maximum display update rate is determined by the operator selected acquisition time window. Note that cathode ray tubes and other display technologies may have an image refresh rate. In this patent document, display update rate is determined by the time required to acquire and plot all the display samples (the display record) and is not related to the refresh rate of the particular display technology.
For additional background, see for example, M. S. Holcomb and D. P. Timm, "A High-Throughput Acquisition Architecture for a 100-MHZ Digitizing Oscilloscope," Hewlett-Packard Journal, Vol. 43, no. 1, February 1992, pp 11-20, R. A. Witte, "Sample Rate and Display Rate in Digitizing Oscilloscopes," Hewlett-Packard Journal, Vol. 43, no. 1, February 1992, pp. 18-19 and S. B. Warntjes, "Sustained Sample Rate in Digital Oscilloscopes," Hewlett-Packard Journal, Vol. 48, no. 2, April 1997, pp. 23-25.
Some instruments in the past have provided manual control over various acquisition and display parameters, thereby permitting operator control of various trade-offs. In addition, several approaches have been taken for automatically providing various optimal measurement parameter values. Prior approaches for automatic optimization typically require a preliminary acquisition of a signal followed by analysis of the preliminary signal acquisition, and then a second acquisition using optimized parameters based on the analysis of the preliminary acquisition. For example, in U.S. Pat. No. 5,375,067 (Berchin), sampling rate is automatically adjusted based on analysis of a preliminary acquisition. In Berchin, a waveform is first digitized using maximum sampling rate and maximum record length. A spectral analysis is performed on the preliminary acquisition. Sampling rate is then set to twice the frequency of the highest frequency component having an amplitude that exceeds a maximum aliasing level, and then the waveform is reacquired with the new sampling rate. In U.S. Pat. No. 5,397,981 (Wiggers), for repetitive signals, sampling rate is constant, and record length is automatically adjusted based on analysis of a preliminary acquisition. In Wiggers, a repetitive signal is acquired and sampled, the time period for one cycle is measured, and the number of samples is adjusted to maintain a constant display time for any signal repetition rate. Note in particular in Wiggers that the acquisition time window is not selected by an operator, but instead the acquisition time window is automatically adjusted to fit a cycle time of a repetitive signal.
There is a general need for automatic optimization of display bandwidth, given a particular operator selected acquisition time window, without requiring a reduction in the display update rate below the maximum rate determined by the acquisition time window. In particular, there is a need for automatic optimization of display bandwidth without requiring a preliminary acquisition and analysis and without requiring a change in the operator selected acquisition time window. In addition, there is a need for automatic optimization of display bandwidth for waveforms other than repetitive waveforms.